Multilevel signals can be used to realize a high bandwidth data transmission in a loss transmission medium such as a cable. For example, pulse amplitude modulation (PAM) is often used to realize multilevel signals. As known, PAM is a form of signal modulation where the data is encoded in the amplitude of a series of signal pulses. It is an analog pulse modulation scheme in which the amplitudes of a train of carrier pulses are varied according to the sample value of the signal. Demodulation is performed by detecting the amplitude level of the carrier at every symbol period.
Amplitudes of signal streams at receivers are different for different cable losses. In order to recovery data correctly, threshold voltages of multilevel signal determination are essential. The demodulation recognizes different data patterns depending on those threshold voltages. Conventionally, the threshold voltages are manually set, which is usually constant and cannot fit all cases with different cable losses. Moreover, manually setting of the threshold voltages may cause false data recognition when cable loss varies.